1. Field of the Invention
This invention relates to the preparation of rigid cellular polymer materials, particularly polyisocyanurate and polyurethane foams, characterized by improved insulating properties. More particularly, the invention relates to an improvement in the foam's insulating capacity by reacting a polyisocyanate prepolymer and polyester polyol containing low free glycol.
2. Description of the Prior Art
The preparation of foams characterized by isocyanurate and urethane linkages is well known in the art. Generally, these foams are prepared by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a catalyst(s). Polyester polyols of many types can be used as the polyol components in the production of these foams.
Because of the relatively high cost of various typical polyols useful in making cellular polymers and because an inexpensive source of polyols is now available from such sources as crude dimethyl terephthalate oxidate residues, recycled poly (ethylene terephthalates), and various other scrap polyester sources, efforts have heretofore been directed to processes for converting these materials to commercially useful polyester polyols. These efforts have led to a number of polyester polyols found to be highly suitable in producing foams of good quality. However, there is still a search for improvements in these polyols and the ways of using them in foam manufacture.
It is also known that polyols, including certain aromatic polyester polyols, can be reacted with isocyanates to form isocyanate-terminated quasi-prepolymers which in turn can be used in the formation of polyurethane and/or polyisocyanurate foams, as disclosed, e.g., in U.S. Pat. No. 4,791,148.
A most important characteristic of rigid foams is their insulating value which is expressed by the k- and aged k- factor. The k-factor is a measurement of the insulating properties (thermoconductivity) and the aged k-factor indicates insulating properties over time. Foams prepared from the above-noted quasi-prepolymers of U.S. Pat. No. 4,791,148 exhibit low initial thermal conductivity values and maintain low thermal conductivity values after prolonged periods of use. It would be most beneficial to provide still further improved formulations for the production of rigid foams having a combination of advantageous properties, including especially good insulating properties, as reflected in low initial thermal conductivity values and low thermal conductivity values after prolonged periods of use.